Claimants battling ACC for entitlements say its $4.9 billion surplus is "ridiculous" and "obscene" given how difficult it has made it for New Zealanders to obtain compensation.

But the Accident Compensation Corporation, which boasts it is in its "best shape ever", says the money will underpin big investments in improving its systems and services following a series of privacy blunders.

Chairwoman Paula Rebstock said the net surplus was $3.6 billion ahead of budget and would allow the corporation to reduce the deficit between its assets and the lifetime cost of every claim on its books from $7.2 billion to $2.3 billion.

Chief executive Scott Pickering said the corporation had surpassed its budget because investment returns on its $24.6 billion reserves were $920 million better than expected, higher interest rates reduced the current value of the future cost of claims by $1.2 billion and improved rehabilitation services reduced estimated future costs by $1.2 billion.

But some claimants told the Herald the surplus came at their expense.

Kaiaua woman Bernadette Dalley is battling ACC for compensation for an injury she says happened in a fall four years ago but which she says ACC has turned down, citing a pre-existing degenerative condition.

She said ACC's surplus was "absolutely ridiculous especially considering they pretty much decline everybody's claims these days".

"The whole system needs to be looked at. I don't think they're allocating funds properly for looking after New Zealanders."

Another client who has battled ACC for years over her claim related to sexual abuse said: "I just think it's bloody obscene.

"I know gains are being made through investments and all the rest of it but at end of the day I think there's been significant cutbacks in how ACC provides services."

ACC Futures Coalition spokeswoman Hazel Armstrong said that while the corporation was financially well managed, "what they haven't been doing for a few years now is investing in claimants".

The coalition yesterday launched its "manifesto" which calls for a series of improvements including better rehabilitation services and creation of an ACC ombudsman.

Mr Pickering said with the corporation's finances "now in the best shape they've ever been in" it was able to "invest substantially in the business to position it for the future".

Keys to surplus

The reasons for ACC's $4.9 billion surplus:*Better performance by its rehabilitation services returning claimants to fitness which reduced estimated future costs by $1.2 billion.*Rising interest rates which reduce the current value of future costs or "outstanding claims liability" by $1.2 billion*Investments generating $920 million more than predicted due to recoveries in local and overseas markets and the performance of its investment team.